1. Field of the Invention
The present invention relates to a toner agitating stepping motor control unit, an image forming apparatus and a stepping motor control method.
2. Description of the Related Art
Conventionally, stepping motors having a feature of being inexpensive and small have been widely used. A stepping motor of this type is disposed to generate a rotational force for rotating an agitation paddle for agitating toner contained in a toner hopper, for example, in an image forming apparatus. Since the agitation paddle is rotated by rotational force generation of the stepping motor in this way, adhesion of the toner contained in the topper hopper is prevented.
In the case of disposing the stepping motor for generating a rotational force to rotate the agitation paddle as described above, the following problem occurs. For example, power is wastefully consumed by the stepping motor. The reason for this problem is described below.
Specifically, an unused state of toner continues for a long time if a turned-off state of the image forming apparatus continues for a long time or if a sleep mode for cutting off power supply to a member with large power consumption (e.g. fixing device for fixing a toner image transferred to a recording medium from an image bearing member) in the image forming apparatus.
The toner tends to adhere if the unused state of the toner continues for a long time in this way. Thus, upon starting image formation using the toner having not been used for a long time, the toner in a state of adhesion needs to be agitated and returned to a pulverized state. Upon starting the agitation of the toner in such a state of adhesion, resistance hindering the rotation of the agitation paddle is large.
If the resistance hindering the rotation of the agitation paddle is large as described above, a step out phenomenon occurs in which the stepping motor cannot be rotated step by step unless a sufficiently large torque is generated in the stepping motor. If such a step out phenomenon occurs, the stepping motor vibrates or rotates in a reverse direction. In order to prevent the occurrence of such a step out phenomenon, it is required to generate a sufficiently large torque in the stepping motor. Thus, the value of an excitation current received by the stepping motor is set at a large value.
However, if the value of the excitation current received by the stepping motor is set at the large value in this way, power consumption increases. Since the excitation current value is normally set to a constant value regardless of the state of adhesion of the toner, the excitation current value remains to be set at the large value even in a state of non-adhesion. Therefore, power is wastefully consumed.
An apparatus for suppressing such a waste of power consumption has been conventionally employed.
For example, the following apparatus is known. Specifically, the apparatus activates a plurality of brushless motors in a stationary state by an open loop control, keeps such a state for a fixed time and then discriminates the state of toner by a closed loop control of the plurality of brushless motors. When the toner is in a state of adhesion, the apparatus independently controls the plurality of respective brushless motors in a time-series manner. Since the apparatus independently controls the plurality of respective brushless motors in the time-series manner in this way, starting sequences of the plurality of respective brushless motors do not overlap. Thus, the apparatus can generate large torques from the plurality of respective brushless motors without increasing the sum total of starting currents of the plurality of brushless motors.
There is also known an apparatus in which the value of a rotational current to be output to a rotary motor of a process cartridge is set at a value larger than the value of a rotational current in normal time when at least the detachment of the process cartridge including an agitation member for agitating the toner is detected. According to this apparatus, power consumption is suppressed in normal time.
The former apparatus controls the plurality of respective brushless motors by the open and closed loops. Thus, expensive motors conforming to the closed-loop control (feedback control) are necessary. Further, in this apparatus, a feedback mechanism enabling the closed-loop control of the motors is necessary. Therefore, cost increases.
The latter apparatus increases the value of the rotational current upon detecting the detachment of the process cartridge including the agitation member for agitating the toner. This apparatus cannot deal with a case where the unused state of the toner continues with the process cartridge mounted.